The normal human heart is a four chamber muscular organ which serves as the main pump of blood of the circulatory system. Systemic venous blood enters the right atrium through the superior and inferior vena cavae; then through the tricuspid valve enters the right ventricle where it is pumped to the pulmonic artery and the lungs through the pulmonary valve. Blood from the lungs enters the left atrium through the four pulmonary veins; then through the mitral valve enters the left ventricle where it is pumped to the aorta and the rest of the body through the aortic valve. The function of these valves is to allow blood flow easily through them in one direction by opening the leaflets and preventing blood from regurgitating back by closing the leaflets. In some individuals one or more valves may not function normally, usually as a result of disease-induced valve damage, degeneration or a congenital defect. Some valves may become stenotic, thus impeding forward blood flow, and some valves may become incompetent, thus allowing blood to backflow through them. Both conditions can lead to life threatening conditions. For the last 35 years, severe valve dysfunction has been treated by replacing the valve with a mechanical prosthesis, or alternatively, with a tissue valve (i.e., a valve of human or animal tissue). Tissue valves have the advantage of low thrombogenicity, therefore they do not need long term anticoagulation, as this is the case for mechanical valves, thus eliminating the potential clinical complications, expense and patient inconvenience. Tissue valves can be categorized as allografts (usually aortic valves from cadavers, also referred as homografts) or xenografts (animal heart valves). In addition some aortic valves have been replaced by autografts (Ross procedure), which is a pulmonary valve from the same patient which in turn is replaced with an allograft (homograft) or a xenograft ("Tissue Heart Valves", ed. by M. I. Ionescu, publisher Butterworth Inc., Boston Mass., U.S.A., 1979, particularly at pp. 146-172). Due to difficulties in sterilization, storage, and availability of multiple valve sizes, allografts (homografts) are not widely used by cardiac surgeons. Ross procedure is a lengthy and technically very demanding operation not widely used either. Xenografts from the other hand are quite commonly used for human cardiac valve replacement. These are either porcine aortic valves or valves constructed from bovine pericardium (W. R. E. Jamieson; J. Card. Surg., 1993;8;89-98). A variety of sizes are readily available. Both types of xenografts have to be first treated with an agent, typically glutaraldehyde, to fix the valve tissue, sterilize it, and decrease its antigenicity. In particular the porcine aortic xenograft has been used for cardiac valve replacement, both as stented (mounted in a frame) or unstented ("Tissue Heart Valves", supra, particularly at pp. 32-34, 107-109, and 177). Because unstented valves minimize turbulence they should reduce thrombosis and embolism. However, they require a more difficult operative procedure for insertion than the stented valves and can only be used in the aortic portion. The porcine aortic valve is a trileaflet valve, but not identical to the human aortic valve. An important distinction is that the porcine aortic valve has a muscle shelf which extends into one of the valve cusps (the right-coronary cusp). This muscle shelf prevents the right coronary cusp from completely opening, thereby partially obstructing the flow. This obstruction is accentuated in small diameter valves as for example in those of children. Attempts have been made to compensate this problem by either trying to enlarge a patients aortic annulus (the portion of the heart in which the valve is seated) so that a porcine aortic valve having a diameter greater than that of the patient's aortic valve could be used, or by developing a technique in which the right coronary cusp of the porcine aortic valve has been replaced with a non-coronary cusp from another porcine aortic valve. However, such techniques require additional manipulations of the patient's aortic annulus or the porcine aortic valve, with their attendant difficulties and expense. Bovine pericardial valves cannot be used in the mitral position due to their high thrombogenicity and structural failure (C. G. Duran, in "Replacement Cardiac Valves" edited by E. Bodnar & R. Frater, publisher Pergamon Press, Inc. New York 10523 USA, 1991, pp. 277-285) and their manufacture requires a lot of technical manipulations that increase their market cost. Both porcine aortic and bovine pericardial valves start to fail within five years from their implantation and eventually all require replacement in approximately ten years (G. I. Grunkemeier et al., Curr. Probl. Cardiol. 1992; 17;362-368). The use of porcine pulmonary valve as was suggested by Ross (U.S. Pat. No. 5,352,240, issued in 1994) has not been widely accepted by cardiac surgeons, as the pulmonary leaflets are very thin and friable and soon fail after the initial implantation. The Wain bioprosthetic cardiac valve (United Kingdom Patent Application No. 8303479, Publication No. 2 136533A, filed by Wain in 1983) represents a valve too cumbersome to manufacture, without any obvious advantage over the existing bioprosthetic valves and without durability. New animals have been tested such as Kangaroo (C. Weinhold et al., Z. Kardiol. 75, Suppl. 2, pp.251-253, 1986), and although promising the limited animal stock will fail to provide enough valves to cover the international market. In addition to the aforementioned limitations, porcine aortic or pulmonary valves are not favored by Muslim patients while bovine pericardial valves are not favored from Buddhists patients and other similar religious groups, thereby excluding a large portion of the world market.